This invention relates to coating compositions and more particularly, coating compositions employed in an one-coat system or as primer coat compositions.
Excellent properties of a fluorocarbon polymer such as low abrasiveness, non-tackiness and high resistances to chemicals and heat, have led to an application of the polymer in a wide variety of fields such as in domestic uses (such as in irons, frypans, etc.), the food industry, the electric industry, the machine industry and like industries. However, because of the inherent non-tackiness, the fluorocarbon polymers are very poor in adhesiveness to all kinds of materials. Thus, this property makes it difficult to coat substrates with such polymers with sufficient adhesion.
In order to overcome this difficulty, several methods have been proposed in which polyarylene sulfide resins (hereinafter referred to simply as PAS), typical of which is polyphenylene sulfide, ##STR1## (HEREINAFTER REFERRED TO SIMPLY AS PPS), are employed due to their high resistance to heat and high affinity for fluorocarbon polymers. For example, in Japanese patent publication No. 31813/1976, there is proposed a method in which a polyphenylene sulfide resin is sprayed over a blasted metal surface and baked to form, in situ, an undercoat layer on which a fluorocarbon polymer topcoat is to be applied. Further, there are also known, from Japanese patent publication No. 12053/1976 and German Offenlegungsschrift No. 2,152,770, primer compositions containing PPS and a fluorocarbon resin as principal components, which are particularly suitable for applying a fluorocarbon polymer topcoat on the primer coat.
A primer coat consisting essentially of PPS has, however, very poor hardness at elevated temperatures (e.g. pencil hardness test result at 180.degree.-200.degree. C. is 2B or less) though it exhibits good hardness at normal temperature (pencil hardness of 4H to 5H). Therefore, an article having such primer and a topcoat encounters a problem in scratch resistance.
When PPS is used in combination with a fluorocarbon polymer, it is not possible to increase the fluorocarbon polymer concentration because of low affinity therebetween. Particularly, when a composition including PPS admixed with a fluorocarbon polymer is used in an one-coat system, the fluorocarbon polymer content is required to be up to 20% by weight. At such a low content, the coat cannot exhibit the good repellency properties inherent to the fluorocarbon polymer. When such composition is used as a primer composition, on the other hand, sufficient adhesion between the primer coat and its fluorocarbon polymer topcoat is not obtained. Moreover, to improve bonding between the primer coat and its substrate, the fluorocarbon polymer content is preferred to be as low as possible which is, however, undesirable with respect to the bond between the primer coat and topcoat.
Further, when a fluorocarbon polymer admixed with PPS is used as a coating composition for an one-coat system, the resulting coat tends to lose its lubricity within a short period of service because of its poor abrasion resistance.